1. Field of the Invention
This invention relates to an optical film using diffraction grating and a display device using the same.
2. Description of the Related Art
Display devices to which such an optical film is applicable include the following.
(1) Transmission type liquid crystal display devices requiring a light source such as a back light unit.
In a transmission type liquid crystal display device, a liquid crystal panel is illuminated at a back side (the side opposite to the side of viewers) or a lateral side with light emitted from a light source such as a back light unit or an edge light unit and a pattern produced on the liquid crystal panel is displayed by emitted light.
With such a display device, an optical film such as a diffusion film or a prism film is used to control illuminating light so that it may irradiate the entire surface of the liquid crystal panel uniformly or in a given direction.
When a back light unit is used, a simple diffusion film will simply diffuse light isotropically so that light cannot be converged to a necessary area of observation to make it impossible to utilize light effectively. On the other hand, a prism film has disadvantages such as a large thickness and being unfoldable.
(2) Reflection type liquid crystal display devices not requiring the use of a light source such as a back light unit.
A reflection type liquid crystal display device comprises a reflector at a rear side of the liquid crystal panel (the side opposite to the side of the viewers) and does not require any particular light source. It is for utilizing environmental light (room illuminations, sun light and/or other external light) to display a pattern produced on the liquid crystal panel.
A display device of this type utilizes any of the optical films listed below in order to control a view area (an area in which display light can be properly sensed by the visible sense) of the viewers.
(a) A diffusion film arranged on a front surface (at the side of the viewers) of the liquid crystal panel in order to scatter reflected light to be used as display light. The diffusion film preferably does not scatter incident light to the liquid crystal panel and but scatters light reflected by the back surface thereof to emit toward the viewers from the liquid crystal panel as display light.
(b) A reflection film arranged on the rear surface (at the side opposite to the side of the viewers) of the liquid crystal panel in order to control the direction and area of reflection of reflected light to be used as display light. While scattering reflector panels (metal plates having an undulated surface) are mainly used as reflection films, they have been partly replaced by hologram films in recent years.
Known reflection type liquid crystal display devices utilizing a hologram film as reflector panel include those disclosed in the following patent documents.
(1) Jpn. Pat. Publication (KOKAI) No. 56-51772
(2) Jpn. Pat. Publication (KOHYO) No. 8-505716
(3) Jpn. Pat. Publication (KOKAI) No. 9-152586
(4) Jpn. Pat. Publication (KOKAI) No. 9-222512
(5) Jpn. Pat. Publication (KOHYO) No. 9-510029
Various types of hologram are known and reflector panels comprising a hologram film show different characteristics depending on the type of hologram used therein.
The patent document (1) discloses the use of a surface relief type hologram having a diffusion pattern (hologram) of shallow gratings as interference fringes. The fringes provide a low contrast and it is difficult to raise a diffraction efficiency. Therefore, it is difficult for viewers to visually sense a bright display pattern and the sensed colors can change depending on a viewing angle because of a color dispersing effect of hologram.
The patent documents (2) and (3) disclose the use of a volume phase reflection type hologram that can reflect and diffract light only in a limited range of wavelength (and hence only of particular colors) because of its wavelength selectivity. Therefore, it is difficult to provide a bright display pattern that can be sensed over the entire wavelength range of visible light.
The patent documents of (4) and (5) disclose the use of a volume phase transmission type hologram that can provide a bright display pattern over the entire wavelength range of visible light (as known to those skilled in the art in the field of technology and hence will not be described in greater detail). However, it is difficult to make all light coming from a reflection layer arranged on the rear surface to work for diffraction (the reflector panel comprising a volume phase transmission type hologram and a reflection layer) and hence to cause a bright display pattern to be visually sensed because of an angle selectivity of a volume phase transmission type hologram.
Additionally, the volume type holograms recited in the documents (2), (3), (4) and (5) are made of a photosensitive material and hence costly. They are also accompanied by a problem of durability.
The use of hologram is also known for diffusion films described in the above (a). In order to prepare such a diffusion film showing a hologram, a light diffusing object such as frosted glass is photographically recorded on a hologram.
However, when photographically recording a light diffusing object on a hologram, a laser beam is made to irradiate a frosted glass that is to be photographed. Then, a projected pattern formed by light transmitted through or reflected by an object to be photographed (object light) inevitably involves a random noise referred to as a speckle pattern. Since the hologram is produced by recording a pattern formed as a result of interference of the projected pattern (object light) and a reference light, the recorded interference fringes show a poor contrast because of the fluctuations in intensity that are caused by a speckle pattern. Therefore, it is a general practice to improve the diffraction efficiency by using a volume phase type hologram such as a photopolymer.
However, a volume phase type hologram is formed by using a photosensitive material that can hardly withstand environmental changes in terms of temperature and humidity and has a large thickness. Thereby, a volume phase type hologram is colored due to absorption of light and is costly as in the case of the use of a reflector panel.
Meanwhile, Jpn. Pat. Publication (KOKAI) No. 11-287991 proposes a reflection type display device as a solution to the problem that known reflection type display devices show a poor efficiency for utilization of light and a poor intensity of light emitted toward the viewers (to display only a dark image) because light reflected by the scattering reflector panel is scattered in every direction.
The proposed reflection type display device comprises a directional reflector panel arranged on the rear surface of a transmission type display member and adapted to reflect incident light coming from the front side of the transmission type display member in a predetermined direction with a spreading angle smaller than the incident angle and a light diffusion member having a geometrical optics effect and arranged at least one of on the front surface of the transmission type display member and at a position between the rear surface of the transmission type display member and the front surface of the directional reflector panel. With this arrangement, the displayed image can be made sufficiently bright when viewed from a given angle and a wide angle of viewing field can be realized.
Meanwhile, the diffusion member having a geometrical optics effect normally has large dimensions with a large thickness and is heavy and costly, which are problems to be solved. Although a scatter panel (diffusion panel) may have a relatively simple configuration among various diffusion members having a geometrical optics effect, it is difficult to control the light emission range.